1. Field of the Invention
This invention relates to methods of upgrading the products derived from the cracking of residua in petroleum refining, particularly methods involving upgrading non-catalytic hydrocracked residua, and especially methods of hydroprocessing hydrocracked residua.
2. State of the Art
Modern requirements for petroleum products place a premium on light, clean burning transportation fuels. Such fuels should be low in sulfur, metals, nitrogen, and aromatic compounds. New requirements place limits on the concentrations of sulfur that can be present in diesel fuel, and requirements for a low smoke point place restrictions on total aromatic compounds allowed in jet fuel. A continuing problem for refiners is producing as much valuable light transportation fuels from crude as possible that meet all relevant specifications.
A particular problem has always been the treatment of residua, the portion of a distilled crude left in the pot after distillation, residua usually being defined as the portion that boils at greater than 560.degree. [C. (1050.degree. F.). Residual are heavy and contain most of the material that degrades the quality of petroleum, for example, metals and sulfur, as well as high molecular weight polynuclear aromatic compounds. High quality light crudes that produce less residua are becoming more scarce in the world, and the heavy crudes remaining tend to make more residua when refined. For example the tar sands of Canada, heavy Mayan crude, Venezuelan crude, and Arabian heavy all produce an abundance of residua when processed. Consequently, refiners increasingly have to face the problem of how to upgrade more residua into a commercial product. It is important that as much residua be turned into naphtha, jet fuel, diesel, and other light transportation fuels as possible.
One method for upgrading residua is shown in U.S. Pat. No. 4,851,107 issued to Kretschamar et al. That process teaches that a fuel, for example, jet fuel (boiling range 150.degree. C.-355.degree. C. (300.degree. F.-520.degree. F.)), is produced by catalytically hydrocracking the entire residua fraction and then subjecting most of the hydrocrackate product to hydroprocessing under severe conditions. The heaviest portion of the hydrocrackate is not hydroprocessed at all, but is combined with the treated lighter portion. Then the combined product is refined as a synthetic crude to produce the fuel products.
However, the treatment described in U.S Pat. No. 4,851,107 presents several problems. First, the heavier portion of the hydroprocessed fraction tends to be cracked during hydroprocessing under severe conditions. This results in the production of large concentrations of light sulfur, nitrogen, and aromatic components, fragments derived from the heavier components of the feed, being included in the lighter boiling fractions. Therefore, the final jet fuel product may not meet the quality jet fuel specification of including no more than 20 vol.% aromatic content. If the hydrotreating conditions are severe enough the quality jet fuel specification may be met, but at the price of creating a naphtha fraction that has too much sulfur and nitrogen to be a suitable reformer feedstock. A reformer feedstock should have less than one part per million of both sulfur and nitrogen.
Second, the hydrocrackate contains components of widely varying molecular weights and boiling points. Therefore, the conditions for hydroprocessing most of the various components of the hydrocrackate cannot be optimized. Consequently, portions of the hydrocrackate feed can be "over" processed, destroying desired components, whereas other portions may not be processed enough to produce the desired products. Furthermore, the extremely severe temperatures and pressures required to upgrade the hydrocrackate to meet the quality jet fuel specification are generally expensive, making the process less economical. Finally, combining an unhydrotreated fraction with a hydrotreated fraction tends to introduce more aromatic components into the final products.
Accordingly catalytically hydroprocessing the entire hydrocracked residua has many drawbacks. It results in an expensive process that yields a product that, while boiling in the jet fuel range, does not meet quality jet fuel aromatic specifications. Clearly, a process that produces a better quality jet fuel from residua is needed, preferably one that is more economical to operate.